1. Field of the Invention
The present invention relates to an optical disk comprising a printed layer on a signal readout surface side of the disk, for displaying material such as the content of the recorded information, and in particular relates to an optical disk for which reproduction is possible using light of any wavelength within the visible light region (380 nmxcx9c800 nm), and moreover for which the visibility of visual information can be ensured.
2. Description of the Related Art
In conventional optical disks, typically a printed layer is provided for displaying visual information such as figures and symbols so that the content of the recorded information can be readily understood.
For example, in the case of single sided recording reproduction type optical disks such as CDs (compact disks), a printed layer, which is formed by methods such as screen printing or offset printing, is provided onto the protective layer positioned on the disk surface reverse to the information signal readout surface, and hence almost the entire surface of the disk can be used for displaying figures and symbols.
In contrast, in the case of double sided recording reproduction type optical disks, onto which larger volumes of information are able to be recorded, the regions of the disk on which a printed layer can be provided are limited to those portions outside of the information recording regions such as the very narrow region close to the central portion of the disk, so as not to obstruct the readout of the information signal. Consequently the surface area which can be used for the display of figures and symbols has been very restricted.
In order to resolve the above problem, attempts have been made to enlarge the display area for visible information on double sided recording reproduction type optical disks, and examples include the inventions disclosed in Japanese Unexamined Patent Publication No. 9-81964, Japanese Unexamined Patent Publication No. 6-55887, and Japanese Unexamined Patent Publication No. 8-273201.
The inventions above comprise two optical disk base bodies, each of which is formed by sequentially laminating an information signal layer of a translucent reflective film, a protective layer, and a printed layer onto an optical disk substrate, and which are then bonded together with the printed layers facing each other. Consequently, the information signal layer has a predetermined transparency, and the printed layer is visible from the disk substrate side of each disk base body which functions as the information signal readout surface, through the information signal layer and the protective layer.
With such a construction, a large surface area for the printing and display of figures and symbols can be ensured on both sides of the disk, without obstructing the readout of the information signal, even with double sided recording reproduction type optical disks.
The wavelength of the light used for reproduction, which is currently about 650 nm, is now on the trend towards shorter wavelengths because this enables further increases in the recording volume. As a result, an optical disk for which reproduction is possible with the light of any wavelength within the visible light region has been much sought after. However, the inventions disclosed in the aforementioned Japanese Unexamined Patent Publication No. 9-81964, Japanese Unexamined Patent Publication No. 6-55887, and Japanese Unexamined Patent Publication No. 8-273201 are unable to generate good reproduction with the light of a shorter wavelength than the wavelength of currently employed light. Moreover, also for the visibility of the visual information, adequate visibility cannot be obtained.
In order to achieve good reproduction, it is necessary for the recording layer of the translucent reflective film to have a comparatively high reflectance in the wavelength region of the laser light used for reproduction, thereby enabling the generation of a reproduction signal of sufficient signal strength. Consequently, a recording layer of an optical disk for which reproduction is possible with light of any wavelength within the visible light region of 380xcx9c800 nm, is required to vary little in terms of reflectance with variations in the wavelength of the light, that is, have a reflectance which displays a low wavelength dependence, and also have a reflectance which exceeds a certain value. On the other hand, it is known that when the visual information is viewed through the translucent reflective film of the recording layer, projection of the visible light reflected back at the recording layer and irregular reflections by the pits, in addition to attenuation of the visible light when passing through the recording layer, decrease the contrast of the dark/bright pattern on the printed layer, thereby lowering the visibility. Then, particularly for the translucent reflective film which displays a low wavelength dependence of the reflectance, the wavelength dependence of the optical attenuation of the translucent reflective film becomes a factor which affects the visibility of the visual information.
The present invention takes the above factors into consideration, with an object of providing an optical disk for which information reproduction are good for reproduction light of any wavelength within the visible light region and for which the visibility of visible information can also be ensured.
In order to achieve the above object, an optical disk according to a first aspect of the present invention comprises a transparent disk substrate, a recording layer formed from a translucent reflective film which is layered on pits based on signal information provided on the disk substrate, and a printed layer which is layered on the recording layer and on which is printed visible information, and is constructed so that the visible information of the printed layer is visible from the disk substrate surface through the recording layer, wherein the optical characteristics of the recording layer obtained through the disk substrate are such that for light of a wavelength within the visible light region, the difference between the maximum reflectance and the minimum reflectance is 25% or below, and the minimum reflectance is within the range 20xcx9c65%, and moreover wherein the difference between the maximum optical absorptance and the minimum optical absorptance is 25% or above.
With such a construction, the recording layer formed from a translucent reflective film is able to acquire a reproduction signal of sufficient strength for light of any wavelength within the visible light region, thereby enabling good reproduction, while also ensuring sufficient visibility of the visible information.
Furthermore with an optical disk of the present invention, preferably the recording layer utilizes a translucent reflective film material for which the optical absorptance either increases or decreases in moving from the short wavelength side to the long wavelength side within the visible light region. Specifically, materials such as silicon, an alloy incorporating silicon as a main component, a compound of silicon with either carbon or nitrogen, or a mixture of silicon and compound of silicon with either carbon or nitrogen should preferably be used.
Furthermore, with an optical disk of the present invention, an optical buffer layer formed from a light transmissive material may also be provided between the recording layer and the printed layer, and by so doing the electrical characteristics can be improved even further.
With an optical disk of the present invention, if the printed layer is constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information, then by selecting a suitable background color, the visibility of the visible information can be further improved. In such a case, if the second printed layer is formed of a single color by so-called contact printing, then the printing process is relatively simple.
In order to apply the present invention to a double sided recording reproduction type optical disk, an optical disk for which the optical characteristics of the recording layer are the type of characteristics according to the first aspect of the present invention may be used for at least one of a first optical disk and a second optical disk, with the first optical disk and the second optical disk then being bonded together with the respective substrates thereof facing outwards, thereby providing visible information on only one side of the double sided optical disk. Alternatively optical disks for which the optical characteristics of the recording layer are the type of characteristics according to the first aspect of the present invention may be used for both a first optical disk and a second optical disk, with the two disks then being bonded together with the printed layers facing each other, thereby providing visible information on both sides of the double sided optical disk. In either case, if an opaque adhesive layer is provided between the first optical disk and the second optical disk to bond the two disks together, then the visibility of the visible information is able to be improved.
With a double sided recording reproduction type optical disk of the present invention, preferably the recording layer, which achieves optical characteristics of the type according to the first aspect of the present invention, utilizes a translucent reflective film material for which the optical absorptance either increases or decreases in moving from the short wavelength side to the long wavelength side within the visible light region, and is constructed from a material such as silicon, an alloy incorporating silicon as a main component, a compound of silicon with either carbon or nitrogen, or a mixture of silicon and compound of silicon with either carbon or nitrogen. Furthermore, an optical buffer layer formed from a light transmissive material may also be provided between the recording layer and the printed layer. Moreover, the printed layer may also be constructed from a first printed layer for printing visible information, and a second printed layer for printing a background color for the visible information. In such cases, the second printed layer may be formed of a single color by so-called contact printing.